The BGS Lexicon of Named Rock Units — Result Details

Cook Formation

Typically, dominantly grey siltstones and silty mudstones with streaks of very fine-grained and well sorted sandstones at some levels, calcite cemented locally. Occasional thin beds of very fine- to coarse-grained, argillaceous, micaceous and chloritic sandstone occur locally (e.g. Well 3/4-8).

Definition of Lower Boundary:

The lower boundary is typically defined by a downward change from siltstone (Cook Formation) to mudstone (Burton Formation) and a distinct down-section increase in gamma values and decrease in velocity. However, locally, where the Cook Formation is less silty or where the Burton Formation is more sandy, log signatures in the boundary interval are not clear.

Definition of Upper Boundary:

The upper boundary is defined by a downward change from mudstones (Drake Formation) to siltstones with subordinate sandstones (Cook Formation) and a down-section decrease in gamma values and, generally, an increase in velocity.